This invention relates to clamps for sealing the instrumentation ports associated with nuclear reactor systems. More particularly, the present invention relates to high quality clamps for maintaining a proper seal at the interface between reactor vessel head penetrations and the thermocouple instrument columns.
Due to the risks associated with operating a nuclear power plant, the design and quality standards associated with nuclear reactor equipment are extremely high and stringent. Accordingly, problems which are capable of straight forward solution in a non nuclear environment are difficult and demanding in the context of a nuclear reactor facility. For example, it is generally required in many industrial settings to monitor the pressure, temperature, and other parameters of various operating equipment. In the environment of a nuclear power plant, this can be extremely dangerous due to the potential for escape of radioactive materials. Accordingly, it is imperative in these situations that the instrumentation used to make such measurements be precisely designed to prevent such leaks.
While the possible escape of radioactive material from a nuclear power plant is minimized by the containment building surrounding the nuclear steam supply system, the working conditions inside the containment building are potentially hazardous even during normal plant operation. This is particularly true during refueling of the reactor when high background and high airborne particulate radioactivity exists in the containment building. Safety regulations set maximum dose limits for the presence of workers in these locations during plant operation and refueling. Many of these same locations are also hazardous during refueling due to the high ambient temperatures. In many situations, these locations are also not easily accessible and a safe work platform is not available. It is accordingly desirable to provide clamping apparatus which are quickly and easily assembled so as to minimize worker exposure to such hazardous conditions. Such quick and efficient repair and/or replacement of instrument port clamps is also highly desirable from the economic point of view since it minimizes the down time of the nuclear plant and hence the cost of providing replacement electricity.
In order to more clearly understand the present invention, one typical instrumentation port interface is revealed in FIG. 1. As revealed by this illustration, a lower conduit or flange 10 is coupled or otherwise mounted to the vessel (not shown) whose parameter is to be measured while the upper conduit or flange 11 is coupled or otherwise mounted to the parameter measuring assembly (not shown). In the particular application of a nuclear power plant, the lower portion of flange 10 is threaded and welded onto the head penetration. Flanges 10 and 11 are generally tubular in shape and have upper and lower surfaces respectively which are designed to engage one another in a sealing manner with respect to gasket 12. In order to effectively compress gasket 12 and seal the interface between flanges 10 and 11, it is desirable for a clamping apparatus to exert axially pressure on flanges 10 and 11. For the instrument port interface shown in FIG. 1, the pressure exerted on each flange should be directed towards the interfacing end of that flange. That is, the clamping apparatus should exert an upward axial force on flange 10 while exerting a substantially equal and axially downward force on flange 11. In this way, the interface between flanges 10 and 11 is properly sealed by gasket 12.
The seal between flange 10 and flange 11 is an important safety consideration in the design of a nuclear power plant reactor. It will be appreciated by those skilled in the art that such flange interfaces are generally located in regions of the plant having a high radioactivity level and high process temperatures. Because of these special circumstances, high quality clamps capable of sealing the interface between flanges 10 and 11 are not only desirable but necessary. In some applications, it is desirable to construct such clamps from high strength material. In addition, it is highly critical to worker safety that the clamping apparatus used to seal such interface be quickly and easily installed and removed.
One heretofore used clamping apparatus, generally designated as 20, is shown in FIG. 2. The clamping apparatus 20 consists of three essentially identical body members 13A, 13B, and 13C. Each body member spans an arc of approximately 110.degree.. An interbody gap 15 of about 10.degree. exists between the body members. Each end of the body members 13A, 13B and 13C contains a flanged portion which is used to attach the body members together. A cap screw 14 (as shown) or other holding means is passed through the flanged ends and holds the body members in a generally. ring-shape while the clamp is assembled on flanges 10 and 11.
The use of the clamp 20 on a nuclear reactor vessel instrumentation port interface as shown in FIG. 1 will now be described. Due to its configuration and weight, the clamp 20 of FIG. 2 is generally brought to the instrument port interface in disassembled form. At least two workers are then generally required to assembly clamp 20 in situ around the outer portion of the interface between flanges 10 and 11. Workmen only have access to flanges 10 and 11 from radially outside the reactor vessel head because of the cooling shroud and other equipment permanently installed thereabove. The specified procedure for operation of the heretofore used clamping apparatus requires the use of an axial loading device which seats the gasket prior to the application of the clamp. Such axial loading devices are generally cumbersome and heavy, making the installation thereof extremely difficult. The requirement of this axial loading device also restricts the work space available and therefore complicates the assembly of clamp 20. Once the axial loading device is properly positioned, the interbody gaps 15 must be carefully adjusted so as to be substantially equivalent in order to achieve generally uniform contact and pressure on the flanges 10 and 11, and to minimize cap screw shank bending. The cap screws 14 are generally torqued to about 100 ft/lb Torquing to this extent may require relatively long torquing systems. It should be noted that, in many applications, over torquing of the cap screws 14 may result in overcompression of gasket 12. For many gaskets, overcompression has a serious detrimental impact on the sealing capacity of the gasket. Prior art clamping apparatus generally used space limiters between the flanges in order to prevent such overcompression of the gasket. It is apparent from the above description that the procedures and apparatus required for the assembly of clamp 20 and other prior clamping devices are thus time consuming and present a large potential for improper installation. The above disadvantages are even more pronounced when it is considered that such a clamp must be installed in awkward and precarious positions requiring workers to be tethered by ropes and/or other safety gear and that workers are required to wear cumbersome gear such as masks, heavy gloves, and radiation suits with respirators.
While the use of articulated clamps to overcome some of the disadvantages described above has been known, the heretofore articulated clamps did not completely overcome the problems and difficulties associated with use of such clamps in a nuclear power plant environment. For example, the heretofore used articulated clamps did not solve the serious problem of possible gasket compression and as a result required the use of space limiters. While such space limiters may have been adequate for the intended purpose, their use is prevented in many nuclear power plants systems due to existing space restrictions and existing instrument port flange configurations.